The development of predictive models for the reductive transformation of nitroaromatics requires further clarification of the effect of environmentally relevant variables on reaction kinetics and the identification of readily available molecular descriptors for calculating reactivity. Toward these goals, studies were performed on the reduction of a series of monosubstituted nitrobenzenes in Fe(II)-treated goethite suspensions. The energy of the lowest unoccupied molecular orbital, ELUMO (B3LYP/6-31G*,water), of the nitrobenzenes was capable of explaining 99% of the variability in the rates. Results of experiments in which the surface area loading of ferric oxides was systematically varied indicate that (i) the reactivity of mineral-surface-associated Fe(II), Fe(II)surf, toward the reduction of p-cyanonitrobenzene (CNNB) decreased in the order hematite > goethite > lepidocrocite > ferrihydrite and (ii) the surface density of Fe(II)surf did not play a crucial role in determining the observed reactivity trend. CNNB was reduced in Fe(II)-only control experiments in a pH range of 7.28-7.97 with a pH dependency consistent with the transformation of Fe(II) to Fe(OH)3 or related oxides. The pH dependency of the reduction of CNNB in Fe(II)-treated ferric oxide suspensions (pH 6.1-7.97) could be accounted for by the oxidation of Fe(II)surf, forming an Fe(III) oxide.

Sorption of aromatic amines to sediments and soils can occur by both reversible physical processes and irreversible chemical processes. To elucidate the significance of these sorption pathways, the sorption kinetics of aniline and pyridine were studied in resaturated pond sediment. Aniline and pyridine behaved quite differently in the sediment-water systems. The sorption kinetics of pyridine were quite fast, reaching equilibrium within 1-2 h. In contrast, the sorption kinetics of aniline were characterized by a rapid initial loss of aniline from the aqueous phase followed by a much slower rate of disappearance. The rapid initial sorption of aniline upon respiking after an equilibration period of 200 h, and results of sorption kinetic studies as a function of substrate concentration, demonstrated that sorptive sites were not being saturated at the nominal concentration of aniline. Sequential extraction of a sediment treated with 14C-labeled pyridine and aniline suggested that pyridine was bound primarily through a reversible cation-exchange process, whereas aniline sorbed through both cation-exchange and covalent binding processes. At longer reaction periods sorption became increasingly dominated by covalent binding. The reaction kinetics for the slow, irreversible sorption of aniline appeared to be limited by the reactivity and/or availability of covalent binding sites. The initial rate and extent of aniline sorption was pH dependent (sorption increased with decreasing pH). At pH values above the pKa of aniline, sorption kinetics for the slower, irreversible loss of aniline were independent of pH.

Uncertainty still exists regarding the role(s) of natural organic matter in the reduction of chemicals in anoxic environments. This work studied the effect of Suwannee river humic acid (SRHA) on the reduction of nitrobenzenes in goethite suspensions by Fe(II) species. The pseudo-first-order rate constant for the reduction of p-cyanonitrobenzene (k(CNNB)) was different for the first 3 half-lives in systems where Fe(II)aq and dissolved SRHA were equilibrated in reverse orders with goethite in suspensions. k(CNNB) and the reduction capacity of the system having SRHA added after Fe(II)aq was equilibrated with goethite was lower than that of the system for which the components were added in the reverse order. SRHA decreased the reduction capacity of the former system by oxidizing and/or complexing the surface-associated Fe(II), Fe(II)(surf), and/or hindering the access of CNNB to Fe(II)(surf). The log k(CNNB) increased linearlywith increasing concentrations of Fe(II)aq, which decreased as a result of increasing concentrations of SRHA in the system. Different k(CNNB)'s were observed for systems in which Fe(II)aq was equilibrated with goethite/SRHA suspensions for 24 and 48 h, suggesting sorbed SRHA oxidized and/or complexed Fe(II)aq. Findings suggest the concentration of Fe(II)aq and accessible Fe(ll)(surf) will influence the reduction rates of nitroaromatics in anoxic environments.